Device and method for measuring medium-high temperature thermal conductivity of material by taking electron beam as heat source

Abstract

The invention discloses a device and method for measuring high-temperature thermal conductivity in a material with an electron beam as a heat source, and relates to the technical field of thermal conductivity measurement, the measuring device comprises a pressing plate and a cooling plate, and a uniform heat flow piece, a sample piece and a standard piece are sequentially arranged between the pressing plate and the cooling plate in the direction from the pressing plate to the cooling plate; temperature measuring holes are formed in the sample piece and the standard piece; a through hole is formed in the middle of the pressing plate and used for heating the uniform heat flow piece through electron beams. A heat insulation sleeve is arranged between the pressing plate and the cooling plate. The uniform heat flow piece, the sample piece and the standard piece are all located in the heat insulation sleeve. According to electron beam heating, the material is impacted through the high-energy electron beam, kinetic energy is converted into internal energy of the material, a heating assembly is not needed, and the problems existing when an existing joule type heating assembly heats a sample piece are solved; in addition, thermal expansion of the material caused by temperature change and linear / nonlinear change of the thermal conductivity are considered, so that measured data of the thermal conductivity of the material are more reliable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of thermal conductivity measurement, and in particular to a device and method for measuring high-temperature thermal conductivity of materials using an electron beam as a heat source. BACKGROUND

[0002] Thermal conductivity is one of the basic thermal physical parameters of materials. In the aerospace, steel and metallurgical industries, materials often serve in a medium-high temperature environment, and accurate knowledge of their thermal conductivity at medium-high temperatures is crucial for thermal design. Thermal conductivity measurement methods can be divided into transient methods and steady-state methods. Transient methods obtain thermal diffusivity by solving transient heat conduction differential equations, and the measurement time is relatively short, but the thermal conductivity needs to be calculated in combination with the specific heat capacity and density, which increases the measurement error. Steady-state methods directly obtain thermal conductivity through Fourier heat conduction law, and the measurement time is relatively long, but the principle is simple and the accuracy is high, and it is still one of the important means for measuring thermal conductivity.

[0003] Currently, when measuring the thermal conductivity of materials at medium-high temperatures using the steady-state method, a Joule heating assembly is usually used to contact the sample to provide heat flow. The heating assembly is complex in structure and is generally composed of an electric heating wire embedded in a heating plate with a corresponding pre-set groove according to a specific arrangement. The material of the heating assembly needs to have properties such as easy processing, high temperature resistance, and high thermal conductivity, which limits the selection range of materials and in turn restricts the upper temperature limit of the steady-state method for measuring thermal conductivity. Common heating wires and heating plates are made of materials such as nickel and copper alloys. In a medium-high temperature environment, the heating wire is prone to oxidation and embrittlement, leading to heating circuit breakage. The heating plate is prone to thermal distortion problems such as overall warping, groove deformation, and micro-cracks due to thermal expansion, causing distortion and fluctuations in the heating heat flow. The above problems superimpose each other, making it difficult for the mechanical and thermal properties of the common heating assembly to be maintained at medium-high temperatures, making it difficult for the temperature measurement upper limit to exceed 1000 K and restricting the stability of the heat flow during measurement.

[0004] The purpose of the present application is to solve the problems of traditional steady-state method for measuring high-temperature thermal conductivity of materials, and to provide a new method and device for measuring high-temperature thermal conductivity of materials with high precision. SUMMARY

[0005] Based on the technical problems existing in the background technology, the present application provides a device and method for measuring high-temperature thermal conductivity of materials using an electron beam as a heat source, which improves the upper limit, precision and stability of the measurement of high-temperature thermal conductivity of materials.

[0006] The application provides a material high-temperature thermal conductivity measuring device with an electron beam as a heat source, which comprises a pressing plate and a cooling plate, the pressing plate and the cooling plate are connected through fasteners, a uniform heat flow piece, a sample piece and a standard piece are sequentially arranged between the pressing plate and the cooling plate in a direction from the pressing plate to the cooling plate, and the sample piece and the standard piece are both provided with temperature measuring holes; a through hole is formed in the middle of the pressing plate and is used for heating the uniform heat flow piece by an electron beam; a heat insulation sleeve is further arranged between the pressing plate and the cooling plate, and the uniform heat flow piece, the sample piece and the standard piece are all located in the heat insulation sleeve.

[0007] Preferably, a heat insulation gasket is arranged between the uniform heat flow piece and the pressing plate.

[0008] The application further provides a measuring method of the material high-temperature thermal conductivity measuring device with an electron beam as a heat source, and the measuring method comprises the following steps:

[0009] S1: calculating the distance L of the temperature measuring section when the standard piece and the sample piece reach a thermal steady state r 、L s and the cross section diameter D r 、D s ;

[0010] wherein the thickness and the diameter of the standard piece when reaching the thermal steady state can be expressed as:

[0011]

[0012]

[0013] the thickness and the diameter of the sample piece can be expressed as:

[0014]

[0015]

[0016] wherein the subscripts "r" and "s" respectively represent the standard piece and the sample piece; L and D respectively represent the distance and the diameter (m) of the temperature measuring section when reaching the thermal steady state; L0 and D0 respectively represent the distance and the diameter (m) of the temperature measuring section when being initially; T t and T b respectively represent the temperatures (K) of the upper and lower temperature measuring sections of the material when reaching the thermal steady state, T0 represents the initial temperature (K) of the material, is the linear thermal expansion coefficient (K -1 ) of the material at the average temperature of the initial and the thermal steady state;

[0017] S2: calculating the heat flow Q r 、Q s through the thickness direction of the standard piece and the sample piece;

[0018]

[0019]

[0020] wherein T t-r , T b-r represent the temperatures of the upper and lower temperature measuring sections of the standard material in thermal steady state; k r represents the thermal conductivity of the standard material; is T t-r , T b-r the average temperature of the standard material thermal conductivity;

[0021] S3: The thermal conductivity of the sample is derived by the finite difference method or the polynomial fitting method to the thermal flow Q s of the sample.

[0022] The finite difference method is simple in principle, but is more reliable when the thermal conductivity of the sample varies linearly with temperature or the difference between T t-s and T b-s is small. The polynomial fitting method is complex in principle, but can directly obtain the nonlinear relationship between the thermal conductivity of the material and the temperature, and to some extent, expand the temperature range.

[0023] Preferably, the calculation method of the finite difference method is:

[0024]

[0025] wherein T t-s , T b-s represent the temperatures of the upper and lower temperature measuring sections of the sample material.

[0026] Preferably, the method steps of the polynomial fitting method are as follows:

[0027] S31: The nonlinear relationship between the thermal conductivity of the sample and the temperature is described by a polynomial to obtain:

[0028]

[0029] wherein A0, A1, …, A n-1 are undetermined coefficients;

[0030] S32: Assuming there are m groups of experimental data, the calculation formula of the jth group of experimental data is

[0031]

[0032] Let:

[0033]

[0034] wherein j = 1, 2, …, n;

[0035] Then the m groups of calculation formulas are written in matrix form as follows:

[0036]

[0037] S33: the matrix is solved to obtain A0, A1,..., An n-1 , thereby obtaining a nonlinear relationship between the thermal conductivity of the material and the temperature.

[0038] The beneficial technical effects of the present application are as follows:

[0039] The present application uses an electron beam for non-contact heating to measure the high-temperature thermal conductivity in a material, avoids problems such as limited selection of heating component materials, easy breaking of a heating circuit, and thermal deformation, and thus improves the upper limit, precision, and stability of the measurement of the high-temperature thermal conductivity in the material; in addition, the present application takes into account the thermal expansion of the material and the linear / nonlinear change of the thermal conductivity due to temperature changes, so that the measured data of the thermal conductivity of the material is more reliable. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 The present application is a schematic structural diagram of a high-temperature thermal conductivity measurement device for a material using an electron beam as a heat source;

[0041] Figure 2 The present application is a temperature rise curve and thermal conductivity measurement result; wherein (a1) TC4 experimental temperature rise; (a2) TC4 thermal conductivity measurement result; (b1) AISI 304 experimental temperature rise; (b2) AISI 304 thermal conductivity measurement result; (c1) pure copper experimental temperature rise; (c2) pure copper thermal conductivity measurement result;

[0042] Figure 3 The present application is a schematic diagram of a traditional heating plate and heating wire in a measurement.

[0043] In the figure: 1-pressing plate, 2-heat insulation gasket, 3-sample piece, 4-water-cooled cover plate, 5-standard piece, 6-temperature measurement hole, 7-uniform heat flow piece, 8-fastening piece, 9-heat insulation sleeve. DETAILED DESCRIPTION

[0044] The present application will be further described below in combination with specific embodiments.

[0045] Embodiment 1

[0046] Reference Figure 1The application discloses a material high-temperature thermal conductivity measuring device with an electron beam as a heat source, which comprises a pressing plate 1 and a cooling plate 4, the pressing plate 1 and the cooling plate 4 are connected through fasteners 8, a uniform heat flow piece 7, a sample piece 3 and a standard piece 5 are sequentially arranged between the pressing plate 1 and the cooling plate 4 in the direction from the pressing plate 1 to the cooling plate 4, and the sample piece 3 and the standard piece 5 are both provided with temperature measuring holes 6.

[0047] The electron beam is emitted by an electron gun, and the electron gun is composed of a tungsten filament, a cathode, an anode, a focusing component and a deflection coil. In the system, the electrons generated by the tungsten filament are accelerated in the electric field formed by the anode and the cathode, and then a high-energy-density electron beam is formed through the focusing lens. The electron beam is shot to the test main body (the uniform heat flow piece 7, the sample piece 3 and the standard piece 5) under the action of the deflection magnetic field to realize heating. The vacuum environment is maintained by two-stage air exhaust equipment of a mechanical pump and a molecular pump, so as to inhibit the convective heat exchange and ensure that the electron beam is stably incident.

[0048] In the embodiment, the uniform heat flow piece 7 is a tungsten column. The tungsten column is introduced for two aspects. Firstly, the steady-state method has a higher requirement for uniform heating, and different heating modes of the electron beam have limitations. The point heating fixes the electron beam to the position to be heated to complete the heating, and the operation is simple, but the beam spot energy is in a Gaussian distribution, and it is difficult to meet the requirement of the steady-state method for uniform heating. The scanning heating realizes approximate uniform heating by adjusting the electron beam residence time, the scanning field frequency and the scanning range, but the operation is complex, accurate positioning is difficult to realize, and the reconstructed temperature field still has a deviation (about 2%~4%). The tungsten column has a high thermal diffusivity (6.7×10 -5 m 2 / s), can homogenize the electron beam energy and simplify the operation. Secondly, when the electron beam heats the electrically insulating material, the electric charge is accumulated on the surface of the electrically insulating material, the electron incidence is blocked and the heating stability is reduced. In the steady-state method measurement, it usually takes several hours or even several days to heat the material to a thermal steady state, and the influence of the electric charge is more significant. The tungsten column has a high electrical conductivity (1.79×10 7 S / m), and the accumulated electric charge can be led into the ground through a wire, so that the range of the device suitable for the sample piece is expanded. The standard piece is made of AISI304 and is used for calibrating the heat flow passing through the sample piece in the axial direction. AISI304 has a lower thermal expansion coefficient (1.42×10 -5 K -1 ) and a higher melting point (1670 K), can stably work at a high temperature, and the thermal conductivity data has a low dispersion in different sources, so that the standard piece is a good standard sample. In order to avoid the corner effect, the test main body is arranged in a cylindrical shape.

[0049] The heat insulation sleeve made of aluminum silicate fiber material and the low-emissivity material foil laid on the inner and outer walls of the heat insulation sleeve are alternately arranged to form a multi-layer heat insulation structure to cover the test body, so as to block the heat loss to the environment, thereby meeting the requirement of the steady-state method for one-dimensional heat transfer.

[0050] In addition, a heat insulation gasket 2 is arranged between the uniform heat flux member 7 and the pressing plate 1, the heat insulation gasket is made of zirconia, is arranged between the upper pressing plate and the tungsten column, and is used for inhibiting heat transfer from the tungsten column to the upper pressing plate, so as to avoid that the upper pressing plate is overheated to affect the pressing performance. The integrated pressing-water cooling component is composed of the pressing plate, the steel bolt, the water cooling cover plate and the water cooling plate base (not shown in the figure). The pressing plate and the water cooling cover plate are made of AISI304, the pressing force is applied to the test body through the bolt connection, so as to fix the structure and reduce the contact thermal resistance of the test body, and then the lateral heat loss of the test body is reduced. The water cooling plate base made of aluminum is connected with the upper water cooling cover plate through the bolt, so as to provide a stable water cooling environment and shorten the time for the device to reach a thermal steady state.

[0051] The K-type armored thermocouple (Kepson) and the high-temperature thermal conductive glue (YK8960 of Yikun Glue Industry) are not marked. The highest temperature measured by the thermocouple is 1273.15 K, and the thermocouple is inserted into the temperature measuring hole and directly contacts the material to measure the temperature. The thermal conductivity of the high-temperature thermal conductive glue is 1.5 W / (m·K), which is applied to the contact surfaces of the uniform heat flux member 7, the sample 3 and the standard piece 5, so as to optimize the thermal contact condition and reduce the lateral heat loss of the uniform heat flux member 7, the sample 3 and the standard piece 5. The thickness is determined according to the actual situation. It should be noted that the standard piece and the upper cover plate of the water cooling plate are directly matched through mechanical processing to avoid the contact thermal resistance. The heat insulation gasket is in direct contact between the upper pressing plate and the tungsten column, and a large contact thermal resistance is maintained, so as to inhibit the heat transfer from the tungsten column to the upper pressing plate, and prevent the upper pressing plate from being overheated to affect the pressing capacity. The heat insulation sleeve is also in direct contact with the upper cover plate of the water cooling plate, and a large contact thermal resistance is maintained, so as to limit the heat exchange between the heat insulation sleeve and the water cooling plate, prevent the temperature drop of the heat insulation sleeve, and increase the lateral heat loss of the uniform heat flux member 7, the sample 3 and the standard piece 5.

[0052] Example 2

[0053] A measurement method of a material high-temperature thermal conductivity measurement device with an electron beam as a heat source, the thermal conductivity measurement device is as described in Example 1, and the method steps are as follows:

[0054] TC4 (7.0-28.4 W / (m·K)), AISI304 (14.8-33.5 W / (m·K)) and pure copper (330-400 W / (m·K)) were chosen as the samples in the order of metal / non-metal and thermal conductivity from low to high. The heating mode was point heating. The acceleration voltage was set to 24.34 kV. The beam spot radius was set to 1 mm. The softening deformation of each sample occurred at 1273 K, 1073 K and 1073 K, respectively. To prevent the sample from softening deformation and causing unstable assembly during measurement, and to take into account the highest temperature resistance of the thermocouple (1273.15 K), the beam current range was set to 3.27-17.08 mA, 2.44-6.44 mA and 3.27-12.47 mA, respectively. After the system reached quasi-steady state, the temperature range of TC4, AISI304 and pure copper was 484.12-1260.91 K, 485.47-1006.11 K and 555.87-1034.83 K, respectively, and a typical temperature rise process is shown in Fig. 1. Figure 2

[0055] When measuring high-temperature thermal conductivity of materials using the steady-state method, the dimensional change caused by thermal expansion cannot be ignored. To simplify the calculation, the relationship between the linear thermal expansion coefficient of the material and the temperature is approximated as a linear relationship, and the error introduced by this assumption is negligible. The thickness and diameter of the standard piece at thermal steady state can be expressed as:

[0056] (1)

[0057] (2)

[0058] The thickness and diameter of the sample can be expressed as:

[0059] (3)

[0060] (4)

[0061] In equations (1)-(4), subscripts "r" and "s" represent the standard piece and the sample, respectively; L and D are the distance and diameter of the temperature measurement section at thermal steady state (m); L0 and D0 are the initial distance and diameter of the temperature measurement section (m); T t and T b are the temperatures of the upper and lower temperature measurement sections of the material at thermal steady state (K), T0 is the initial temperature of the material (K), is the linear thermal expansion coefficient of the material at the average temperature of the initial and thermal steady states (K -1 ).

[0062] Assuming that the heat flux through any cross-section in the z direction of the standard piece is equal after reaching thermal steady state, the heat flux through the thickness direction of the standard piece and the sample is calculated as follows: ​

[0063] (5)

[0064] In the formula, k r represents the thermal conductivity of the standard material (W / (m·K)).

[0065] For AISI304 material, its thermal conductivity has a significant linear relationship with temperature, and for the purpose of simplifying calculation, k r is set as a monomial first-order function. The device is quasi-one-dimensional heat transfer along the z direction of the sample and the standard, so the heat flow through the sample in the z direction can be expressed as:

[0066] (6)

[0067] In the formula, is the thermal conductivity of the standard material at T t-r , T b-r average temperature (W / (m·K)).

[0068] The heat flow Q s of the sample is derived by the finite difference method, and the thermal conductivity of the sample is obtained:

[0069] (7)

[0070] The finite difference method is more reliable when the thermal conductivity of the sample changes linearly with temperature or the difference between T t-s and T b-s is small. In addition, because the thermal conductivity of common materials changes nonlinearly with temperature within a certain temperature range, the heat flow Q s of the sample is derived by using the polynomial fitting method based on the least squares method, and the thermal conductivity of the sample is obtained. The specific expression is as follows:

[0071] (8)

[0072] In the formula, A0, A1, …, A n-1 are all undetermined coefficients.

[0073] Assuming that there are m sets of experimental data, the calculation formula of the jth set of experimental data is:

[0074] (9)

[0075] Let:

[0076] (10)

[0077] In the formula, j = 1, 2, …, n;

[0078] Then the m sets of calculation formulas are written in matrix form as:

[0079] (11)

[0080] The matrix can be solved to obtain A0, A1, …, A n-1 , thereby obtaining the nonlinear relationship between the thermal conductivity of the material and the temperature. The polynomial fitting method can directly obtain the nonlinear relationship between the thermal conductivity of the material and the temperature, and to some extent, expands the temperature range.

[0081] In the present embodiment, the L 0-s of AISI304, TC4 and pure copper are 13.00 mm, 13.00 mm and 12.50 mm respectively, and the L 0-r are all 20.00 mm. The parameters of T t-s , T b-s and L 0-s are substituted into equation (7) and equation (11), and the obtained thermal conductivity measurement results of TC4, AISI304 and pure copper are shown in parts (b2), (c2) and (d2) of FIG. 2 respectively, and the deviations of each sample from the literature are -3.22%~4.54%, -3.13%~2.64% and -3.81%~1.44% respectively, which are in good agreement. Figure 2 In the present study, the thermal conductivity uncertainty u kr / k r of the standard sample is 4%. The measurement uncertainty u D of the thickness and diameter of the temperature measurement section is u l =0.01 mm (the accuracy of the screw micrometer). The temperature measurement uncertainty u T / T is 0.24% (provided by Kepson Measurement and Control Technology Co., Ltd.). The linear thermal expansion coefficient uncertainty u α / α of the fused quartz, TC4, AISI304 and pure copper is 5%, 8%, 3% and 10% respectively (provided by the COMSOL material database). The u ks / k s of TC4 and AISI304 are 4.3%~5.1% and 4.6%~7.5% respectively. The u ks / k s of pure copper is significantly increased to 41.2%~66.7%, which is because the thermal conductivity of pure copper is relatively high, resulting in a smaller temperature difference of the sample, which amplifies the temperature measurement uncertainty of the thermocouple itself. Considering that the test results of pure copper are in good agreement with the literature (-3.81%~1.44%), the actual uncertainty should be less than the calculated value. Although the embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present application, the scope of the present application is defined by the appended claims and their equivalents, and all should be included within the protection scope of the present application.

Claims

1. A device for measuring the high-temperature thermal conductivity of materials using an electron beam as a heat source, characterized in that, The device includes a pressure plate (1) and a cooling plate (4), which are connected by fasteners (8). A uniform heat flow element (7), a sample (3) and a standard part (5) are arranged sequentially between the pressure plate (1) and the cooling plate (4) along the direction from the pressure plate (1) to the cooling plate (4). Temperature measuring holes (6) are provided on the sample (3) and the standard part (5). A through hole is opened in the middle of the pressure plate (1) for heating the uniform heat flow element (7) with an electron beam. A heat insulation sleeve (9) is also provided between the pressure plate (1) and the cooling plate (4). The uniform heat flow element (7), the sample (3) and the standard part (5) are all located inside the heat insulation sleeve (9).

2. The high-temperature thermal conductivity measuring device for materials using an electron beam as a heat source according to claim 1, characterized in that, A heat insulation gasket (2) is provided between the uniform heat flow element (7) and the pressure plate (1).

3. A method for measuring the high-temperature thermal conductivity of a material using an electron beam as a heat source, wherein the thermal conductivity measuring device is as described in claim 1 or 2, characterized in that... The steps are as follows: S1: Calculate the distance L between the temperature measuring sections when the standard part and the sample reach thermal steady state. r L s and cross-sectional diameter D r D s ; S2: Calculate the heat flow Q through the thickness direction of the standard part and the sample. r Q s ; In the formula, T t-r T b-r These represent the temperatures of the upper and lower thermometric sections of the standard material in a thermally steady state; k r Thermal conductivity of representative standard materials; For T t-r T b-r Thermal conductivity of standard parts at average temperature; S3: The heat flux Q of the sample is determined by the finite difference method or polynomial fitting method. s The thermal conductivity of the sample was obtained through derivation.

4. The measurement method of the high-temperature thermal conductivity measuring device in materials using an electron beam as a heat source according to claim 3, characterized in that, The calculation method of the finite difference method is as follows: In the formula, T t-s T b-s These represent the temperatures of the upper and lower temperature measurement sections of the sample material, respectively.

5. The measurement method of the high-temperature thermal conductivity measuring device for materials using an electron beam as a heat source according to claim 3, characterized in that, The steps of the polynomial fitting method are as follows: S31: The nonlinear relationship between the thermal conductivity of the sample and temperature is described by a polynomial, yielding: In the formula, A0, A1, ... A n-1 All are undetermined coefficients; S32: Assuming there are m sets of experimental data, the formula for calculating the j-th set of experimental data is: make: In the formula, j = 1, 2, ..., n; Then, the m sets of calculation formulas can be written in matrix form as follows: S33: Solving the matrix will yield A0, A1, ..., A n-1 Thus, a nonlinear relationship between the thermal conductivity of the material and temperature is obtained.

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